1. Field of the Invention
The present invention relates, in general, to copolyester resins and articles using the same, and more particularly, to a copolyester resin composed mainly of terephthalic acid, ethyleneglycol, 1,4-cyclohexanedimethanol, polyethyleneglycol bisphenol-A and a multifunctional monomer, and a molded article using the same.
2. Description of the Related Art
In recent years, a polyester resin copolymerized with 1,4-cyclohexanedimethanol has been commercially used in the fields of packaging materials, molded articles, films, etc.
In this regard, U.S. Pat. Nos. 5,340,907 and 5,681,918 disclose use of a polyester resin copolymerized with 1,4-cyclohexanedimethanol and a preparation method thereof.
The copolyester resin is generally prepared by subjecting terephthalic acid, ethyleneglycol, and 1,4-cyclohexanedimethanol to esterification and then polycondensation in the presence of a stabilizer and a catalyst.
Although melt viscosity of the thus prepared polyester resin is a level suitable for sheet molding by use of an injection process and calender rolls, it is relatively low for use in profile extrusion and extrusion blow molding of large bottles which have a predetermined cross-sectional area without the need of the calender rolls.
Likewise, U.S. Pat. Nos. 4,217,440 and 4,983,711 disclose a method of increasing melt viscosity by using an additive with a multifunctional group, such as a trifunctional group, upon extrusion blow molding. Specifically, upon preparation of a copolyester resin composed of terephthalic acid, ethyleneglycol and 1,4-cyclohexanedimethanol, a multifunctional monomer, such as trimellitic acid, pentaerythritol or the like, is used in the amount of 0.05-0.5 mol %, to increase melt viscosity. Thereby, the copolyester resin has a high molecular weight due to branching effects, and thus, is advantageous in terms of high melt viscosity. However, the above copolyester resin suffers from increased melt pressure upon molding.
With reference to FIG. 1, there is illustrated viscosity variation versus shear rate of copolyester resins. As shown in this drawing, a copolyester resin (B) containing a multifunctional monomer as an additive increases in molecular weight due to the branching effects, and hence, has viscosity in a low shear rate region, that is, melt viscosity, higher than that of a general copolyester resin (A). However, the above copolyester resin has higher viscosity than that of the general copolyester resin even in the shear rate region similar to internal conditions of an extrusion blow molding machine, whereby it increases in melt pressure upon molding. Under high melt pressure, RPM cannot increase to improve productivity and to shorten a work period, thus extending cycle time. Further, a molding temperature cannot sufficiently decrease because of a narrow molding temperature range, thereby not realizing high melt viscosity and melt strength.